// convert #hex notation to rgb array
var parseColor = function (hexStr) {
  return hexStr.length === 4 ? hexStr.substr(1).split('').map(function (s) { return 0x11 * parseInt(s, 16); }) : [hexStr.substr(1, 2), hexStr.substr(3, 2), hexStr.substr(5, 2)].map(function (s) { return parseInt(s, 16); })
};

// zero-pad 1 digit to 2
var pad = function (s) {
  return (s.length === 1) ? '0' + s : s;
};

var gradientColors = function (start, end, steps, gamma) {
  var i, j, ms, me, output = [], so = [];
  gamma = gamma || 1;
  var normalize = function (channel) {
    return Math.pow(channel / 255, gamma);
  };
  start = parseColor(start).map(normalize);
  end = parseColor(end).map(normalize);
  for (i = 0; i < steps; i++) {
    ms = i / (steps - 1);
    me = 1 - ms;
    for (j = 0; j < 3; j++) {
      so[j] = pad(Math.round(Math.pow(start[j] * me + end[j] * ms, 1 / gamma) * 255).toString(16));
    }
    output.push('#' + so.join(''));
  }
  return output;
};

// try if it works
console.log(gradientColors('#00ff00', '#ff0000', 100));

// 泥萌的新需求
console.log(gradientColors('#000', '#fff', 100, 2.2));